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Australopithecus (Latinaustralis "southern", Greek πίθηκος pithekos "ape") is a
genus of extincthominids, made up of the gracile
australopiths, and formerly also included their larger
relatives, the robust
australopiths (which are now given their own genus).
The genus Australopithecus is closely related to the human
genus Homo, and may be
ancestral to it.

Evolution

Map of the fossil sites of the early
Australopithecines in Africa

Reconstruction of Australopithecus
afarensis

Gracile australopiths shared several traits with modern apes and
humans, and were widespread throughout Eastern and Northern Africa by a time between 3.9 and
3.0 million years ago. The earliest evidence of fundamentally
bipedal hominids can be observed at the site of Laetoli in Tanzania. This
site contains hominid footprints that are remarkably similar to
those of modern humans and have been dated to as old as 3.7 million
years. Until recently, the footprints have generally been
classified as australopith because that had been the only form of
pre-human known to have existed in that region at that time;
however, some scholars have considered reassigning them to a yet
unidentified very early species of the genus
Homo.

Australopithecus
anamensis, Australopithecus afarensis
and Australopithecus
africanus are among the most famous of the extinct
hominins. A. africanus used to be regarded as ancestral to
the genus Homo (in particular Homo erectus). However, fossils assigned to the genus Homo have been
found that are older than A. africanus. Thus, the genus
Homo either split off from the genus
Australopithecus at an earlier date (the latest common
ancestor being A. afarensis or an even earlier form,
possibly Kenyanthropus
platyops), or both developed from a yet possibly unknown
common ancestor independently.

According to the Chimpanzee
Genome Project, both human (Ardipithecus, Australopithecus
and Homo) and chimpanzee
(Pan troglodytes and
Pan paniscus) lineages
diverged from a common ancestor about 5 to 6 million years ago, if
we assume a constant rate of evolution. It is theoretically more
likely for evolution to happen more slowly, as opposed to more
quickly, from the date suggested by a gene clock (the result of
which is given as a "youngest
common ancestor", i.e., the latest possible date of diversion.)
However, hominins discovered more recently are somewhat older than
the molecular clock would theorize.
Sahelanthropus
tchadensis, commonly called "Toumai" is about 7
million years old and Orrorin
tugenensis lived at least 6 million years ago. Since
little is known of them, they remain controversial among scientists
since the molecular clock in humans
has determined that humans and chimpanzees had an evolutionary
split at least a million years later. One theory suggests that the
human and chimpanzee lineages diverged somewhat at first, then some
populations interbred around one million years after
diverging.

Morphology

Mrs. Pless, a female
Australopithecus Africanus

The brains of most species of
Australopithecus were roughly 35% of the size of that of a
modern human brain. Most species of
Australopithecus were diminutive and gracile, usually
standing between 1.2 to 1.4 m tall (approx. 4 to 4.5 feet). In
several variations of Australopithecus there is a considerable
degree of sexual dimorphism,
meaning that males are larger than females. Modern hominids do not
appear to display sexual dimorphism to the same degree —
particularly, modern humans display a low degree of sexual
dimorphism, with males being only 15% larger than females, on
average. In Australopithecus, however, males can be up to 50%
larger than females. New research suggests that sexual dimorphism
may be less pronounced than this, but there is still debate on the
subject.

Species variations

Although opinions differ as to whether the species aethiopicus,
boisei and robustus should be included within the
genus Australopithecus, the current consensus in the
scientific community is that they should be placed in a distinct
genus, Paranthropus, which is
believed to have developed from the ancestral
Australopithecus line. Up until the last half-decade, the
majority of the scientific community included all the species shown
in the box at the top of this article in a single genus. However,
Paranthropus was morphologically distinct from
Australopithecus, and its specialized morphology also
implies that its behavior was quite different from that of its
ancestors.

Evolutionary role

Cast of the skeleton of Lucy, an
Australopithecus afarensis

The fossil record seems to indicate that Australopithecus
is the common ancestor of the distinct group of hominids, now
called Paranthropus (the
"robust australopiths"), and most likely the genus Homo
which includes modern humans. Although the intelligence of these
early hominids was likely no more sophisticated than modern apes,
the bipedal stature is the key evidence which distinguishes the
group from previous primates who are quadrupeds. The morphology of
Australopithecus upsets what scientists previously
believed, namely, that large brains preceded bipedalism. If A.
afarensis was the definite hominid which left the footprints
at Laetoli, it strengthens the notion that
A. afarensis had a small brain but was a biped. Fossil
evidence such as this has made it clear that bipedalism far
predated large brains. However, it remains a matter of controversy
how bipedalism first evolved millions of years ago (several
concepts are still being studied). The advantages of bipedalism
allowed hands to be free for grasping objects (e.g. carrying food
and young), and allowed the eyes to look over tall grasses for
possible food sources or predators. However, many anthropologists
argue that these advantages were not large enough to cause the
evolution of bipedalism.

A recent study of primate evolution and morphology noted that all
apes, both modern and fossil, show skeletal adaptations to upright
posture of the trunk, and that fossils such as Orrorin tugenensis indicate
bipedalism around 6 million years ago, around the time of the split
between humans and chimpanzees indicated by genetic studies. This
suggested that upright, straight-legged walking originally evolved
as an adaptation to tree-dwelling. Studies of modern orangutans in Sumatra showed that
these apes use four legs when walking on large stable branches,
swing underneath slightly smaller branches, but are bipedal and
keep their legs very straight when walking on multiple small
flexible branches under 4 cm. diameter, while also using their
arms for balance and additional support. This enables them
to get nearer to the edge of the tree canopy to get fruit or cross
to another tree.

It is suggested that the ancestors of gorillas and chimpanzees
became more specialised in climbing vertical tree trunks, using a
bent hip and bent knee posture which matches the knuckle-walking
posture they use for ground travel. This was due to climate changes
around 11 to 12 million years ago that affected forests in East and
Central Africa so that there were periods when openings prevented
travel through the tree canopy, and at these times ancestral
hominids could have adapted the upright walking behaviour for
ground travel. Humans are closely related to these apes, and share
features including wrist bones apparently strengthened for knuckle-walking. The view that human
ancestors were knuckle-walkers is now questioned since the anatomy
and biomechanics of knuckle-walking in chimpanzees and gorillas are
different suggesting two separate evolutions that happened after
the last common ancestor with the human linerage. Further
comparative analysis with other primates suggests these wrist bone
adaptations support a palm based tree walking.

Radical changes in morphology took place before gracile
australopiths evolved; the pelvis structure and feet are very
similar to modern humans. The teeth have small canines, but
australopiths generally evolved a larger post-canine dentition with
thicker enamel.

Most species of Australopithecus were not any more adept
at tool use than modern non-human primates, yet modern African
apes, chimpanzees, and most recently gorillas, have been known to use simple tools (i.e.
cracking open nuts with stones and using long sticks to dig for
termites in mounds), and chimpanzees have
been observed using spears (not thrown) for hunting.However, some
have argued that A.
garhi used stone tools due to a loose association of this
species and butchered animal remains.

Diet

In a 1979 preliminary microwear study of Australopithecus fossil
teeth, anthropologist Alan Walker theorized that robust
australopiths were largely frugivorous.
However, newer methods of studying fossils have suggested the
possibility that Australopithecus was omnivorous. In 1992, trace element studies of the
strontium/calcium ratios in robust australopith fossils suggested
the possibility of animal consumption, as they did in 1994 using
stable carbon isotopic analysis. Australopithecus mainly ate fruit,
vegetables, and tubers.